This invention relates to semiconductor devices of the kind having heat radiating plates and encapsulated in a resin package, as well as electronic circuit boards having such a device mounted thereon.
Among semiconductor devices of the kind containing inside a resin package a semiconductor chip which radiates a large amount of heat during operation such as power ICs, there are those provided with heat radiating plates for efficiently allowing the heat from the semiconductor chip to escape to the exterior. FIG. 13 shows an example of prior art semiconductor device D of this kind structured in a dual in-line form and having lead pins and heat radiating plates in a so-called surface-mountable form. Its resin package 1, containing a semiconductor chip inside, has a specified thickness in the vertical direction and an approximately rectangular shape when seen from above. A plurality of lead pins 2 and a heat radiating plate 3 protrude from each of its two side surfaces, each of the lead pins 2 being connected by wire bonding to a specified one of pads on the semiconductor chip inside the resin package 1, and the heat radiating plate 3 contacting the semiconductor chip inside the resin package 1. The lead pins 2 and the heat radiating plate 3 are each bent into the shape of a crank, having a planar attachment part 21 (also referred to as "the pin-attaching part") or 31 (also referred to as "the plate-attaching part") formed at the outer end (distal from the package 1). Correspondingly, wiring patterns 5 corresponding to the lead pins 2 and heat radiating patterns 6 are formed on a circuit board, say, by pattern-etching a copper foil.
Conventional heat radiating plates were rectangular, having the same width both at the base and at the outer edge. Likewise, heat radiating patterns corresponding to these heat radiating plates were conventionally made in a simple belt-like form having a uniform width matching that of the associated heat radiating plate. In other words, side edges 32 of the heat radiating plate 3 and side edges 62 of the heat radiating pattern 6 were parallel to each other as seen from above, and as shown in FIG. 14.
When an electronic component of a surface mounting type is mounted to a circuit board by a solder reflow method, cream solder is applied first by a printing process on selected portions of the wiring on the circuit board. After the electronic component is positioned on the circuit board, it is introduced into a heating oven. The solvent agent within the cream solder is evaporated, and the solder component is melted to provide electrical connections between the lead pins of the electronic component and the wiring pattern on the circuit board. As the solder is cooled and solidified, a mechanical connection is completed between the lead pins and the pattern.
Suppose this process is carried out for the surface mounting of the prior art semiconductor device D shown in FIG. 13 by soldering. As shown in FIG. 14, cream solder is applied to areas 2A and 3A by printing on each wiring pattern 5 and the heat radiating pattern 6, but the width of the heat radiating pattern 6 is greater than that of the wiring pattern 5, as shown in FIG. 14, because the heat radiating plate 3 is made as wide as possible for improving the heat radiating efficiency while the lead pins 2 are usually made as thin as possible for making the device compact. Thus, the width L.sub.2 of the solder-applying area 3A printed on the heat radiating pattern 6 is greater than the width L.sub.1 of the solder-applying area 2A printed on the wiring pattern 5.
Let us assume that L.sub.2 is four times as great as L.sub.1. This means that four times as much cream solder is deposited by printing on the heat radiating pattern 6 than on each of the wiring patterns 5. This, however, does not mean that the total length of the circumference around the cream solder applied in the area 3A on the heat radiating pattern 6 is four times greater than that around the cream solder applied in each of the areas 2A on the wiring patterns 5. In the example shown in FIG. 14, L.sub.2 is only about 2.5 times as long as L.sub.1, and this causes a problem as follows.
When the semiconductor device D is positioned on the circuit board after cream solder is applied by printing, as described above, and the solder component of the cream solder is melted in a heating oven, the planar attachment parts 21 and 31 of the lead pins 2 and the heat radiating plate 3 compress the melted solder from above due to the weight of the semiconductor device D. This compressive force has the effect of pushing the melted solder out of the space between the attachment parts 21 of the lead pins 2 and the wiring patterns 5 and the space between the planar attachment part 31 of the heat radiating plate 3 and the heat radiating pattern 6. Since the ratio between the amount of the printed solder and the circumferential length around the area printed with solder is greater at the joint for the heat radiating plate 3 than at the joints for the lead pins, relatively more solder will be squeezed out of the printed area on the heat radiating plate 3. As a result, the heat radiating pattern 6 and the adjacent one of the wiring pattern 5 may become shorted by the solder that has been squeezed out. This becomes more likely to occur as the pitch of the lead pins 2 is reduced for producing compact devices.